SOFIA Delivers First Complete Map of Ionized Carbon in the Fireworks Galaxy

Calibration for young distant galaxies is now possible

NGC 6946 the Fireworks Galaxy
NGC 6946, the Fireworks Galaxy. Image credit: ESA/Hubble & NASA, A. Leroy, K.S. Long

Using the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), developed by the University of Stuttgart and installed aboard the flying observatory SOFIA, a team led by Frank Bigiel of the Argelander Institute for Astronomy, or AIfA, at the University of Bonn, Germany, has completed a far-infrared map of the spiral galaxy NGC 6946 revealing the distribution of ionized carbon in this galaxy. These data help to estimate the star-formation rate not only in the nearby universe but also in distant galaxies of the early universe.

The Fireworks galaxy: An ideal laboratory in our galactic neighborhood

The galaxy, NGC 6946, is about twelve million light-years away from Earth and, thus, still located in our cosmic neighborhood. It owes its nickname – “the Fireworks galaxy” – to the fact that 10 supernova explosions have taken place in it in the last 100 years. For comparison, in our home galaxy, the Milky Way, the last supernova happened in the year 1604. NGC 6946 is one of the most gas-rich spiral galaxies in our neighborhood and produces many more new stars than is typical for its size.

A tool for measuring star-formation rate: [CII] 158 µm line

The emission of the singly ionized carbon, [CII], has a wavelength of 158 µm and thus lies in the far-infrared part of the electromagnetic spectrum. It is produced in the interstellar medium when young, newly formed stars heat the gas around them, and the ionized carbon then re-radiates this thermal energy. The strength of the [CII] 158 µm line allows astronomers to determine the energy released and thus the number of young, hot stars. This far-infrared spectral line is an ideal measurement tool for the star-formation rate in a galaxy. This re-radiation process of ionized carbon is also important for cooling the interstellar gas, allowing more stars to form. The Fireworks galaxy converts about six solar masses into stars each year, compared to the Milky Way which converts just one solar mass per year. The Fireworks galaxy is an ideal laboratory to study in detail the relationship between the star-formation rate and the [CII] 158 µm line.

[CII] 158 µm map of the Fireworks Galaxy overlaid on optical multicolor image
[CII] 158 µm map (black contours) of the Fireworks Galaxy overlaid on optical multicolor image from the LBT (Large Binocular Telescope). Credit: [CII] map: Bigiel et al. ApJ 903, 2020; LBT image: D. Paris, V. Testa, LBC team, and D. Thompson, LBTO).

Calibration for distant galaxies

“Our study allows us to break down what fraction different regions of the galaxy contribute to the emission of the [CII] 158 µm line,” explains Frank Bigiel, lead author of the study. 73% of the radiation originates in the spiral arms, where most of the stars also form, 19% in the center of the galaxy, and only 8% from the regions between the arms. This work is helping scientists understand the influence of different environmental conditions, such as temperature and density, on the star-formation rate of a galaxy and thus the strength of this line. “A precise understanding of these relationships in such a nearby spiral galaxy is important to calibrate the [CII] 158 µm line as a ‘measuring instrument”’ and also to be able to use it to determine the star-formation rate in very distant galaxies of the early universe,” Frank Bigiel says. The [CII] 158 µm line is very bright, so up to 1% of the total infrared radiated energy of a galaxy can come from this line alone. This makes it easy to observe even in very distant galaxies. Additionally, because of the cosmic redshift of the wavelengths, it can be observed with radio telescopes. The central question of how star-forming activity in galaxies has evolved over the billions of years since the formation of the universe, producing stars such as our Sun, can thus be answered – at least in part.

Reliable measuring tool

Stars “burn” hydrogen and helium by nuclear fusion to higher-value elements. As stars age, fusing more and more of their hydrogen and helium, their so-called metallicity increases. When massive stars die, they explode and hurl this enriched material back into the interstellar medium, from which more stars can form. The study by Frank Bigiel’s team confirms that the [CII] 158 µm line works excellently as a measurement tool for the star formation rate in regions of varying metallicity: Only under the extreme conditions at the center of the galaxy, which is characterized by immensely turbulent gas flows, does much work lie ahead for researchers to understand the deviant behavior of the [CII] line.

[CII] 158 µm map of the Fireworks Galaxy
[CII] 158 µm map of the Fireworks Galaxy. Credit: [CII] map: Bigiel et al. ApJ 903, 2020

Working on more maps

“FIFI-LS is the only instrument and SOFIA the only observatory that currently allows us to efficiently produce such large maps of ionized carbon,” says Alfred Krabbe, director of the German SOFIA Institute (DSI) at the University of Stuttgart and co-author of the study. “Therefore, at DSI we are also working on comparable maps of other spiral galaxies in our vicinity, such as the galaxy M83 – the Fire Wheel galaxy in the southern sky – to optimize the calibration of the [CII] 158µm line for more distant galaxies.”

Original paper

SOFIA/FIFI-LS Full-disk[CII] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946, F. Bigiel et al., ApJ 903, Nov 2020

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SOFIA is a joint project of NASA and the German Space Agency at DLR. DLR provides the telescope, scheduled aircraft maintenance, and other support for the mission. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.